Spurious Mode Suppression in Quasioptical Waveguides by Lossy Dielectric Sidewall Coatings

The use of lossy dielectric sidewall coatings for spurious Hm0 mode suppression in quasioptical waveguides is studied. It is discussed how to select permittivity, loss tangent, thickness and coating length. A resonance absorption occurs if the electrical thickness of the coating roughly equals ¿/4. With approximations of the propagation constants in the coated waveguide section simple formulas for the resonance frequency and the attenuation below, at and above resonance are derived. A rigorous calculation of the scattering matrix at the junction to empty waveguide controls the approximate formulas. The spurious mode suppression can be improved by placing absorber foil in the evanescent mode region, of a discontinuity. This is shown with a miter elbow by combined scattering matrices and is verified by measurements.